For five years I have lived a strange double life. Well, triple life, really. On one level I am a TV journalist, pinging around the world in search of trouble-makers. Afghanistan, Iraq, Lebanon, Libya. On another, I am a home-maker, helping my wife chip Coco Pops from beneath the dining table and packing lunches for our half-dozen children.

Last and least, I am an amateur academic, who has been commissioned by a think tank to scour dozens of studies in search of something new under the sun of social policy-making. In particular, proof that having a brother or sister is a good thing.

Writing about the virtues of siblinghood might seem a strange hobby for a parent struggling under the weight of a six-strong brood. After all, even Brad and Angelina say they find their six children a struggle. So why do it? The answer lies in an early-evening news-reading shift eight years ago and a round-up of the day’s lesser stories. One of those items concerned the cost of raising a child, which was said to have reached £250,000.

There are many contraceptive forces behind the 10 per cent rise, in the space of a generation, in the number of mainly middle-class British parents having only one child. The housing market, expensive child care, lost career momentum. Our only sibling subsidy – child benefit – has been capped for all and axed for many. But although want of cash is a real disincentive, for the nearly two-thirds of first-time parents who say they cannot afford family expansion, want of accurate information may also be a factor.

Every year, another round of inflation-busting estimates emerges from the press offices of some of our biggest financial services companies. All are looking to secure publicity for their products. Some peddle pseudo-scientific acronyms such as 'COTS’ (Cost Of The Sibling). But none takes into account the economies of scale of a multi-child family. How clothes and toys are bought for one and handed down to another. How the average cost of a child falls as they share everything from household heating to bath water. How parents pay less for play dates if siblings entertain one another. How sibling discounts apply, from school places to theme park tickets.

So, with the help of Swedish researcher Therese Wallin, I set about pulling together data that put a different cost/benefit complexion on the “shall we have another?” conversation.

Some of the most startling literature comes from medical research. It has long been known that siblings – by sharing germs at a young age and mutually priming immune systems – provide some protection against atopic conditions such as hay fever and eczema. But the latest breakthroughs suggest growing up with a brother or sister can also guard against food allergies, multiple sclerosis and some cancers. For reasons that have yet to be fully fathomed, these benefits do not apply to children simply by dint of spending time sharing bugs with other youngsters – as they would, for instance, in day care.

The other “epidemics” of modern childhood, obesity and depression, are also potentially reduced by exposure to siblings. A clutch of major studies from all over the world shows that the more siblings a child has, the thinner they will be. Put simply, siblings help children burn off fat. One American study honed its analysis down to an amazingly precise deduction: with each extra brother or sister, a child will be, on average, 14 per cent less obese. Reductio ad absurdum? We can scoff at such a definitive conclusion, until we realise that no one in medical academia has suggested that having a sibling ever made anyone fatter.

None of this is rocket science. When we compare like with like, regardless of family background, children with siblings tend to enjoy better mental health. Obviously, again, this is to generalise massively. The world is full of jolly singletons. But dig into some of the big data sets out there and unignorable patterns emerge. On experiences on which nation states hold a big corpus of statistics, events such as divorce and death, for example, strong correlations exist.

Cause is not always correlation, but it stands to reason that when parents split up or die, a child will benefit from having a sibling to turn to.That solidarity runs throughout the lifespan. After all, a sibling is for life, not just for childhood.

Indeed, policymakers with an eye to areas beyond elderly care may need to wake up to the shifting sands of family composition. In the late 20th century, the received wisdom among sociologists was that it mattered not a jot to society at large whether more people were sticking to one child. Now that assumption is being questioned. Is the valuable role played by siblings in elderly care factored into the welfare debate? Will an economy with fewer creative middle children be as competitive? How easy will the state find waging war when more parents are reluctant to see their only child march to the front?

More broadly, the last decade has seen a major evolution in academic thinking about siblings. They have ousted parents as being the key driver behind personality development. And where, 30 years ago, academics such as Toni Falbo argued that to be born an only child was to have won the lottery of life, now research is running in the opposite direction.

A slew of reports by serious scholars, such as Prof Judy Dunn of King’s College London, have chipped away at the idea that family size is the product of a consequence-free decision. Researchers have shown that “siblinged” children will have stronger soft skills and keener emotional intelligence than single children. They will be better at gratification deferment (because they have learnt to wait their turn) and hit motor milestones such as walking and talking more rapidly than those without sibling stimulation.

Some of the most recent evidence even suggests that a child with a brother and/or sister will have more evolved language skills and do better at exams. This information is truly revolutionary. For decades, the assumption of academic ideas such as the Dilution Theory has been that less is more.

Have too many children and, as a parent, you will not be able to leverage your resources on to a solitary stellar-achieving child. Indeed, for parents who cannot stop themselves hovering above and over-scheduling their hurried offspring, a sibling for their one-and-only can be the antidote to pushy parenting.

“There is a danger of suffocating a child with too much pressure,” Amy Chua told me in an interview. She is the best-selling author of The Battle Hymn of the Tiger Mother, a parent so determined to wring the last drop of performance out of her children that she would arrange marathon daily violin lessons, even on holiday. Having more than one, said Chua, had blunted her laser-like focus.

Chua’s views on parenting have been heeded by millions. But I also wanted to listen to those anonymous child-care professionals who will, often sotto voce, argue that British children are changing, and not for the better. The kindergarten nanny, for instance, who told me of the game she plays spotting which of her new intake has a brother or sister (she claimed a 90 per cent success rate).

To some only-children and their parents, that will sound the ancient hollow note of animus towards those deemed somehow selfish in not – where possible – having another. In truth, that bigotry has fewer adherents nowadays. People understand that being a one-off parent is a natal no-brainer, a logical response to the economic challenges of parenthood. What, hitherto, they have failed to see, is that modern social science is rewriting the way we see siblings and, yes, it may be worth paying in the short term to benefit in the long term. Either way, the Cost of the Sibling is nothing like as high as some would have you believe.

And what of my own children? How do they feel, providing the material for a sibling laboratory? The eldest, just 14, has already announced that, should she have children, their numbers will be limited.

My wife and I started out similarly sceptical about fecundity. But, having struggled to have a second child, it was hard to shake the mindset that a pregnancy was anything other than a blessing. As our family expanded, necessitating bigger cars and fewer holidays, we took to heart the views attributed to Elizabeth Longford, the historian and Roman Catholic mother-of-eight. Asked why so many, she said that since her children were so different, curiosity drove her to find the limits of genetic diversity.

We find that a big family has unleashed the inner anthropologist in us, too. Some friends flinch at the managed chaos of our home, but my wife and I love the abundance of human interaction. We are the directors of our own daily soap opera.

Colin Brazier is a presenter for Sky News. His book, 'Sticking up for Siblings’, will be published by Civitas on August 22. To order, contact Civitas on 020 7799 6677